February 2020 - Beginners

Re: [Haskell-beginners] Prelude source code
by Doug McIlroy 27 Feb '20

27 Feb '20

> Thank you. Ive also found other references online, but I want to see the > one installed locally in my system. > > I come from Smalltalk where everything is accessible and modifiable. > > Is the Prelude and other base modules distributed in compiled form only? One of the joys of Hugs is that it includes the H98 Prelude, a real vade mecum for both beginners and experts. It is one of the reasons that for everyday use I resort more often to Hugs than to ghci. The modern Prelude, spread over dozens of files, and programmed at a higher level of abstraction, will not be as useful to a beginner as the H98 Prelude is. Nevertheless it's a valuable and fundamental resource. And it's real code, snatches of which one may borrow as a basis for special modification. That is not true of stuff served up by Hoogle. Quite properly, the ghc distribution contains a compiled version of the Prelude. But that doesn't preclude including the source. It would be a public service to do so. ["You can always get it with Cabal" is unreasonably dismissive. Why should beginners have to master another arcane subject just to get a handbook for their newest toy?] Doug McIlroy

1 0

Prelude source code
by Francisco Garau 26 Feb '20

26 Feb '20

I want to see the source code of the Prelude. Is that available in the standard distribution? Any hints in which directory should I be looking? I've installed stack on OSX 10.11 with the below command curl -sSL https://get.haskellstack.org/ | sh Thanks, Francisco

5 6

2D drawing library for Beginner
by Olivier Revollat 14 Feb '20

14 Feb '20

HI, I would like to experiment with 2D graphics with Haskell (to do for example generative art, ...) I found Diagrams library which seem very powerful but I find it a bit overwhelming for beginner, so do you have anything I can start with ? maybe even a game engine .... Thanks ;)

2 2

Hello (First message on the mailing list)
by Olivier Revollat 14 Feb '20

14 Feb '20

Hi everybody, it's my first message on this ML :) I don't know if it's appropriate to post this here but I would like to have some feedback with one of my first Haskell code. I've been inspired by a recent Numberphile video ( https://www.youtube.com/watch?v=HJ_PP5rqLg0) how explain the "Russian Peasant" algorithm to do multiplication (here in a nutshell : https://www.wikihow.com/Multiply-Using-the-Russian-Peasant-Method) So I decided I give it a go in Haskell, here is my solution, I appreciate if you give me some feedback on how to improve this code (make it more "idiomatic Haskell") NB : I apologize if it's not the right place to ask this kind of review ... in that case, where can I post this ? Thanks ! module DivRusse where main :: IO () main = do putStrLn "13 x 12 is" print $ russmul 13 12 russmul :: Int -> Int -> Int russmul a b = let filteredPair = filter (\pair -> (fst pair) `mod` 2 /= 0 ) $ (a,b) : russmulList a b in foldr (\pair acc -> snd pair + acc) 0 filteredPair russmulList :: Int -> Int -> [(Int, Int)] russmulList 1 _ = [] russmulList a b = let a' = a `div` 2 b' = b * 2 in (a', b') : russmulList a' b'

3 4

Memory usage of if-then-else vs guards
by Miguel Angel Ordoñez Silis 14 Feb '20

14 Feb '20

Hí everybody, Earlier today I was testing some code in ghci with ":set +s" enabled. For some reason I changed an if-then-else expressions to guards and I was surprised to find out that the memory usage declined significantly (around 20%). Here is a function call output using if-then-else: λ> inarow1 ls 1 (1.28 secs, 401,690,400 bytes) And here is a function call output using guards: λ> inarow1 ls 1 (1.18 secs, 313,690,576 bytes) I ran both versions many times and this difference was consistent. Could you help me understand why? Thanks Miguel P.S. I don't think it should matter but maybe it does, here is the function definition: inarow1 :: forall a. Eq a => [a] -> Int inarow1 [] = 0 inarow1 (x:[]) = 1 inarow1 ls = aux 0 1 ls where aux :: Int -> Int -> [a] -> Int aux top curr (x:y:[]) = max top $ if x == y then curr + 1 else curr aux top curr (x:xs@(y:ys)) | x == y = aux top (curr + 1) xs | otherwise = aux (max top curr) 1 xs And the list: ls = [1..1000000]

1 0

Need some understanding on this behavior.
by Alexander Chen 05 Feb '20

05 Feb '20

hi, I need some understanding on the following behavior: a = [1..] [x | x <-a, x <= 100] [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100 (note the missing ']' which results in the command not finishing) b = take 1000 a [x | x <-b, x <= 100] [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100] Note, I took more for b just for ease sake. But now the command finishes correctly. Question: why can i not use a lazy list directly in a list comprehension? best,

5 5